As is widely known, in what are known as cans and bottle cans that are filled with refreshment drinks and the like, in order to arouse the desire of a consumer to purchase such drinks, a variety of designs and the like are affixed to the drum portion so that a product identification capability and the like is given to these cans. Conventionally, for example, providing coatings or embossing processing are known as ways of providing such designs.
As far as the latter, i.e., embossing processing, is concerned, a method such as that shown in, for example, Patent Document 1 noted below is known for forming a can body by performing embossing processing on the drum portion of the can base. In this method, there are provided a first rotating body and a second rotating body that are supported so as to be able to rotate around rotation axes that are parallel with each other. The first rotating body is placed on an interior side of a single close-ended cylindrical can base, and the second rotating body is placed on an outer side of the can base. Next, the first and second rotating bodies are moved towards each other and, in a state in which the drum portion of the can base is sandwiched between the outer circumferential surfaces of the respective rotating bodies, the first and second rotating bodies are rotated around their rotation axes.
Note that a DI can that is formed by performing, for example, twisting and ironing processing on a metal plate is used as this can base. Moreover, because this method also has the function of guiding the can base in the radial direction (referred to below as a ‘guiding function’) by making the outer circumferential surface thereof conform to the inner circumferential surface of the can base when the first rotating body is inserted into the interior side of the can base, there is only a small difference between the inner diameter of the can base and the outer diameter of the first rotating body which is typically kept to approximately 0.8 mm.
In recent years, in order to impart an even greater product identification capability to cans and the like, there have been demands for convex embossing processing that protrudes outwards in the radial direction from the outer circumferential surface of the drum portion of a can base.
However, in the conventional can body manufacturing method, in addition to the fact that there is only a small gap between the outer circumferential surface of the first rotating body and the inner circumferential surface of the can base, because it is necessary to form a first convex portion that protrudes outwards in the radial direction from the outer circumferential surface of the first rotating body in order to provide the convex embossing processing on the can drum portion (referred to below as the ‘embossing processed portion’), the aforementioned gap becomes even smaller by the same distance as the height of the protrusion of the convex portion.
Accordingly, when this first rotating body is inserted into the interior side of the can base, there is a possibility that the first convex portion of this rotating body will collide with an aperture end portion of the can base. In addition, after the embossing processed portion has been formed, when the first rotating body is being withdrawn from the interior side of the can base, there is a possibility that the convex portion of the rotating body will become caught on the inner circumferential surface of the embossing processed portion. The problem has accordingly arisen that it is difficult to form this type of embossing processed portion. Furthermore, because the first convex portion which is the outermost portion in the radial direction of the first rotating body functions as a guide portion that guides the inner circumferential surface of the can base, when only a small proportion of the entire outer circumferential surface of the first rotating body is occupied by the first convex portion, it is not possible to sufficiently demonstrate the aforementioned guiding function.
In order to solve the above described problems, the following method may be considered. Namely, a first concave portion that is recessed inwardly in the radial direction is formed in the outer circumferential surface of the first rotating body, and the first convex portion is formed on a bottom surface of this concave portion. In addition, a second convex portion that protrudes outwardly in the radial direction is formed on a portion of the outer circumferential surface of the second rotating body that corresponds to the first concave portion, and a second concave portion that is recessed inwardly in the radial direction is formed in a portion of this second convex portion that corresponds to the first convex portion. When a can drum portion is then sandwiched between these rotating bodies, the portion of the can body that corresponds to the first concave portion and the second convex portion is depressed inwardly in the radial direction so as to form a concave processed portion, and the embossing processed portion is formed in portions corresponding to the first convex portion and the second concave portion.
However, in this method, as a result of both convex and concave embossing processed portions being formed in the can drum portion, there is an increase in the proportion of the overall can drum portion that is occupied by plastically deformed portions. As a result, the problems arise that there is a lowering of the obtainable buckling strength of the can body, breakages occur in the coating film formed on the inner and outer surfaces of the can drum portion, and what is known as blocking may occur when a plurality of can bodies are transported collectively on a transporting conveyor. Furthermore, the problem has also arisen that it has not been possible to reliably prevent the aforementioned first rotating body from becoming caught as is described above when it is extracted from a can body.
Note that, because the above described embossing processing is performed after the coating film has been formed on the inner and outer surfaces of the can base, the coating film is easily damaged during this embossing processing by the convex portions or concave portions or by the convex portions and concave portions that are formed on the outer circumferential surfaces of each of the rotating bodies.
In order to prevent this type of damage to the coating film, for example, as shown in FIG. 9, the size of a convex portion 101a that is formed on the outer circumferential surface of a first rotating body 101 is made smaller than the size of a concave portion 102a that is formed on an outer circumferential surface of a second rotating body 102.
Accordingly, when the drum portion of a can base is sandwiched between outer circumferential surfaces of the rotating bodies 101 and 102, the can drum portion that is positioned between the upright surfaces 101b and 101b of the wall surface forming the convex portion 101a that extend outwardly in the radial direction from the outer circumferential surface of the first rotating body 101, and the inner wall surfaces 102b and 102b of the wall surface forming the concave portion 102a that are opposite the upright surfaces 101b and 101b and extend inwardly in the radial direction from the outer circumferential wall of the second rotating body 102 is stretched in the radial direction in what might be called an unrestrained state.
As a result, when the can drum portion is sandwiched between outer circumferential surfaces of the first and second rotating bodies 101 and 102, the problem arises that abrasion tends to occur easily between surfaces 101c of the first rotating body 101 that are outermost in the radial direction and the upright surfaces 101b. 
Moreover, while the can drum portion is being sandwiched, because it is stretched in an unrestrained state, the unrestrained portion is pulled gently upright in the radial direction from the circumferential surface of the can drum portion and the problem arises that it is difficult to form a well-defined embossing processed portion.
However, in recent years, in order to impart an even greater product identification capability to cans and the like, there have been demands for well-defined embossing processed portions having sharp upright portions in the drum portion of a can base, and there have also been demands for a plurality of embossing processed portions to be formed in a tight grouping in a small area.
However, as is described above, as it is difficult to form this type of embossing processed portions, then if an attempt is made to form even more well-defined embossing processed portions, it is necessary to increase the height of the convex portions 101a and the depth of the concave portions 102a and narrow the width of the convex portions 101a, and to further increase the amount of embossing processing on the outer surface of the drum portion of the can base. In this case, there is a possibility that the convex portions will be bent easily and that tension will cause the coating film formed on the inner and outer circumferential surfaces of the drum portion of the can base to become damaged. As a result, the problem arises that it is even more difficult to form this type of embossing processed portion.
Moreover, if the amount of embossing processing is increased, then there is a corresponding greater amount of flow in the material of the drum portion of the can base during the embossing processing, and there is also a greater amount of tensile deformation in the can drum portion which is in an unrestrained state. This causes the thickness of the can drum portion which includes these areas to become even thinner. As a result, if an attempt is made to form a plurality of well-defined embossing processed portions, then it is necessary to increase the distance between adjacent embossing processed areas, and the problem arises that it is difficult to carry out what is known as fine processing in which a plurality of embossing processed portions are formed in a tight grouping.
Furthermore, the convex portions 101a and the concave portions 102a are typically formed using laser processing, however, if the height and depth of the portions 101a and 102a are increased, there is a corresponding reduction in the processing accuracy. Consequently, the problem has arisen that there is a reduction in the accuracy with which the embossing processed portions are formed.
It should be noted that, there are also demands when carrying out embossing processing on a can drum portion for rectilinear ridgelines to be formed at, for example, approximately 5 or 6 mm intervals, and for a plurality of folded portions having only a small amount of concavity and convexity to be formed adjacent to each other so as to create a fine pattern. This is in order to increase the product identification capability of cans and the like even further.
PATENT DOCUMENT 1: Published Japanese Translation No. 2000-515072 of the PCT International Application